1. Field of the Invention
The present invention generally relates to an electrostatic actuator and, more particularly, to an electrostatic inkjet head having an electrostatic actuator including a diaphragm driven by an electrode located adjacent to the diaphragm with a small gap therebetween. The electrostatic actuator according to the present invention is preferably used for an inkjet head of a high-quality color image printer, a micro-pump or a micro-pressure sensor.
2. Description of the Related Art
As an on-demand type inkjet head, a piezo on-demand type inkjet head and a bubble-jet type inkjet head are popular. The piezo on-demand type inkjet head uses a piezoelectric element to drive a diaphragm which constitutes a fluid chamber storing ink so that a droplet of the ink is discharged from the fluid chamber when the piezoelectric element is driven. The bubble-jet type inkjet head uses a heating element provided in a fluid chamber so as to generate bubbles in ink stored in the fluid chamber by heating the heating element so that the ink in the fluid chamber is discharged by the pressure caused by the bubbles. However, the piezo on-demand type inkjet head and the bubble-jet type inkjet head must be inexpensive and have a very small size and high operating speed. Additionally, a higher density and a higher image quality are also required of such inkjet heads.
In order to achieve the above-mentioned requirements, electrostatic inkjet heads using an electrostatic actuator have been suggested. In the electrostatic inkjet head, an electrostatic actuator is constituted by a diaphragm defining a fluid chamber and an electrode situated adjacent to the diaphragm. Pressure of ink in the fluid chamber is increased so as to discharge the ink from the fluid chamber by displacing the diaphragm by applying an electrostatic force generated by the electrode.
Japanese Laid-Open Patent Application No.5-50601 discloses an electrostatic inkjet head. The electrostatic inkjet head is constituted by an upper substrate made of silicon and a lower substrate. A nozzle, a discharge chamber, an ink cavity a diaphragm and an ink supply opening are provided in the upper substrate. An electrode is provided in the lower substrate. The upper substrate and the lower substrate are joined so that the electrode is opposite to the diaphragm with a small gap therebetween. An electric field is applied between the diaphragm and the electrode so as to generate an electrostatic attractive force.
Additionally, Japanese Laid-Open Patent Application discloses an electrostatic inkjet head having a gap between a diaphragm and an electrode. The gap is 0.05 xcexcm to 2.0 xcexcm so that the inkjet head can have a small size and high density and is driven by a low voltage. The gap is defined by a recess formed in a silicone oxide film (SiO2) by an etching method.
However, in the above-mentioned inkjet heads, it is difficult to maintain a uniform gap when a plurality of gaps are formed within a few millimeters of each other. Additionally, the gap is determined by a depth of etching when a silicon (Si) substrate or a glass substrate is etched. The depth of etching is controlled by an etching time and depends on a temperature of an etchant, concentration of the etchant and the etching time. Since the gap is controlled by controlling the etching time when a part of a silicon oxide film or a glass film is etched, dispersion of the gap is also large.
In additionally, if a glass substrate is used, a very small scratch, which is produced when the glass substrate is polished and cannot be identified by a visual check, is produced. Such a scratch causes a relatively large recess when an etchant enters the flaw.
Additionally, when a protective film is formed by a thin-film forming process, a surface roughness is unsatisfactory (unevenness of a surface is about 2,000 xc3x85), and it cannot be a uniform gap.
In order to overcome the problems identified above, preferred embodiments of the present invention provide an improved and useful electrostatic inkjet head in which a gap between a diaphragm and an electrode is accurately controlled and made to have a uniform size.
In order to achieve the above-mentioned objects, there is provided according to one aspect of preferred embodiments of the present invention, an inkjet head which is constructed to eject a droplet of ink from a pressurizing chamber by pressurizing the ink in the pressurizing chamber by displacing a diaphragm defining a portion of the pressurizing chamber. The diaphragm is preferably displaced via application of an electrostatic force generated by an electrode disposed opposite to the diaphragm. The diaphragm and the electrode are arranged such that there is a predetermined gap between the diaphragm and the electrode. The inkjet head preferably includes a diaphragm substrate having the diaphragm provided therein so that the diaphragm includes a bottom portion of the diaphragm substrate and an electrode substrate connected to the diaphragm substrate so that the electrode faces the diaphragm of the diaphragm substrate, the electrode substrate having a depression arranged such that the electrode defines a bottom of the depression and the predetermined gap is defined by a depth of the depression. The depression is preferably formed by removing an oxidation layer which is formed by selectively oxidizing a portion of the electrode substrate.
Additionally, there is provided according to another aspect of preferred embodiments of the present invention an inkjet head with a similar construction as that of the inkjet head described in the preceding paragraph, the inkjet head including an electrode substrate having the electrode on a top surface thereof and a diaphragm substrate connected to the electrode substrate so that the electrode of the electrode substrate faces the diaphragm provided in the diaphragm substrate, the diaphragm substrate having a depression so that the diaphragm is defines a bottom portion of the depression and a depth of the depression defines the predetermined gap. The depression is preferably formed by removing an oxidation layer which is formed by selectively oxidizing a portion of the diaphragm substrate.
According to preferred embodiments of the present invention, the gap between the electrode and the diaphragm is defined by the depth of the depression formed in the diaphragm substrate or the electrode substrate. The depression is preferably formed by removing an oxidation layer which is formed by selectively oxidizing a portion of the diaphragm substrate or the electrode substrate. That is, the depth of the depression is preferably determined by a thickness of a portion of the oxidation layer, which portion extends from a surface of the diaphragm substrate or the electrode surface. In one preferred embodiment, such a portion of the oxidation layer is preferably about 44% of the overall thickness of the oxidation layer. Since the thickness of the oxidation layer can be accurately controlled, the depth of the depression which is formed by removing the oxidation layer can also be accurately controlled. Accordingly, the gap between the diaphragm and the electrode can be formed with great accuracy and reliability in the preferred embodiments of the present invention. Thus, the inkjet head according to preferred embodiments of the present invention has uniform and reliable characteristics.
Other elements, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.